Any county or municipality or two or more municipalities jointly may raise and appropriate money in the aid of the acquisition, construction, maintenance, repair and improvement of any armory located therein, and to that end may issue bonds payable not more than thirty years after their issue and bearing interest at a rate not exceeding six percent per annum, and may deposit such money and funds and the proceeds of the sale of such bonds with the State Treasurer in the special account provided by section nine of this article to the credit of the proper armory project, and may make such further provisions for the maintenance and improvement of such armory and its joint use with the National Guard as may be deemed necessary: Provided, That whenever the board deems it expedient and in furtherance of the purposes of this article it may purchase and finish armories already built or partly built. Counties and municipalities constructing and financing armories under the provisions of this article or the provisions of article four-a, chapter eight of this code shall convey such armories to the armory board upon payment of all bonds, and interest thereon, issued for the construction or improvement thereof.